1. Field of the Invention
The present invention relates to a semiconductor optical amplifier, and more particularly to a reflective semiconductor optical amplifier suitable for a wavelength-division-multiplexing passive-optical-network (hereinafter, referred to as a “WDM-PON”). The amplifier has high gain even under conditions of low operating current and low threshold current, thereby affording reduced power consumption.
2. Description of the Related Art
From among the WDM-PON light sources being considered for future ultra high speed optical subscriber networks, mode-locked Fabry-Perot lasers with incoherent lights or wavelength-seeded reflective semiconductor optical amplifiers (R-SOAs) have gained recognition, because one of them can be shared by various subscribers and they can be manufactured inexpensively. Yet, the output of the mode-locked Fabry-Perot laser diode (FP-LD) may be unstable, because its locking characteristics become deteriorated when temperature or environmental change modifies the wavelength of oscillation mode. Since the reflective semiconductor optical amplifiers, by contrast, simply amplify and modulate incoherent light assigned to each subscriber, their output power does not change much even though spectrum changes slightly due to external conditions, and relative intensity noise due to gain saturation characteristic is reduced. Accordingly, research has been actively pursued into utilization of the reflective semiconductor optical amplifier.
However, since general semiconductor optical amplifiers have driving current and threshold current higher than those of FP-LDs, the semiconductor optical amplifiers consume a large amount of power. In addition, since the semiconductor optical amplifiers have long length and do not oscillate, they have they exhibit an unfavorable high frequency response characteristic. Reflective semiconductor optical amplifiers having general structures are therefore unsuitable as light sources by which to amplify, modulate, and transmit signals in the WDM-PONs.